I've read over and over that fats should be added after the kneading
is mostly complete, because they inhibit gluten formation. So I
always add butter or oils after letting the dough hydrate and when
the kneading is more than halfway done.
So using the food processor, I whirl together the dry ingredients,
including instant yeast; separately whisk together the liquids, aside
from any oil; all at once add the liquid to the dry and process
briefly; pause 5-10 minutes to let the dough hydrate; knead 20-30
seconds; add the fats; then finish with another 10-20 seconds of kneading.
Easy, quick, and the gluten stays happy.
Diane Brown in St. Louis
http://www.well.com/user/debunix/recipes/FoodPages.html